docs: def set_dextra(self, dextra=None, method='set'):
          """ Set (or update) the dextra dict
  
          dextra is a dict of nested dict
          It contains all extra signal that can help interpret the data
              e.g.: heating power time traces, plasma current...
          Each nested dict should have the following fields:
              't'    : 1D np.ndarray (time vector)
              'data' : 1D np.ndarray (data time trace)
              'name' : str (used as label in legend)
              'units': str (used n parenthesis in legend after name)
  
          Use the kwarg 'method' to set / update the dict
  
          """
          assert method in ['set','update']
          dextra = self._checkformat_inputs_dextra(dextra=dextra)
          if method == 'set':
              self._dextra = dextra
          else:
              self._dextra.update(dextra)